Synchronous Rectification Control for Flyback Converter

Patent metadata

Field	Value
Publication number	US-2018367052-A1
Application number	US-201715627579-A
Country	US
Kind code	A1
Filing date	Jun 20, 2017
Priority date	Jun 20, 2017
Publication date	Dec 20, 2018
Grant date	—

Abstract

A flyback converter includes a primary-side switch connected to a primary-side winding of a transformer and a secondary-side switch connected to a secondary-side winding of the transformer. The flyback converter is operated by controlling the primary-side switch to store energy in the transformer during ON periods of the primary-side switch, switching on the secondary-side switch synchronously with switching off the primary-side switch to transfer energy from the transformer to the secondary side, determining an off time of the secondary-side switch based on a reflected input voltage measured at the secondary-side winding when the primary-side switch is on, accounting for a settling time of the reflected input voltage when determining the off time of the secondary-side switch so that the settling time has little or no effect on the off time, and switching off the secondary-side switch based on the off time.

First claim

1 . A method of operating a flyback converter having a primary-side switch connected to a primary-side winding of a transformer and a secondary-side switch connected to a secondary-side winding of the transformer, the method comprising:
controlling the primary-side switch to store energy in the transformer during ON periods of the primary-side switch; switching on the secondary-side switch synchronously with switching off the primary-side switch to transfer energy from the transformer to the secondary side; determining an off time of the secondary-side switch based on a reflected input voltage measured at the secondary-side winding when the primary-side switch is on; accounting for a settling time of the reflected input voltage when determining the off time of the secondary-side switch, so that the settling time has little or no effect on the off time; and switching off the secondary-side switch based on the off time, wherein the flyback converter comprises a current source configured to provide a charge current for charging a capacitor on the secondary side when the primary-side switch is on and a current sink configured to sink a discharge current for discharging the capacitor when the secondary-side switch is on, and wherein accounting for the settling time of the reflected input voltage when determining the off time of the secondary-side switch comprises: adjusting the discharge current for a present switching cycle of the secondary-side switch by an error current which is based on a comparison of the off time determined for a previous switching cycle of the secondary-side switch to a maximum off time calculated for the secondary-side switch.

2 . (canceled)

3 . The method of claim 1 , wherein switching off the secondary-side switch based on the off time comprises:
switching off the secondary-side switch when the voltage across the capacitor decreases to a first reference value.

4 . The method of claim 1 , wherein adjusting the discharge current for the present switching cycle of the secondary-side switch based on the reflected input voltage as measured at steady-state after the settling time lapses during the previous switching cycle comprises:
comparing the off time determined for the previous switching cycle of the secondary-side switch to a maximum off time calculated for the secondary-side switch; and decreasing the error current for the present switching cycle of the secondary-side switch if the off time determined for the previous switching cycle is less than the maximum off time.

5 . The method of claim 1 , wherein adjusting the discharge current for the present switching cycle of the secondary-side switch based on the reflected input voltage as measured at steady-state after the settling time lapses during the previous switching cycle comprises:
comparing the off time determined for the previous switching cycle of the secondary-side switch to a maximum off time calculated for the secondary-side switch; and increasing the error current for the present switching cycle of the secondary-side switch if the off time determined for the previous switching cycle is greater than the maximum off time.

6 . The method of claim 1 , wherein adjusting the discharge current for the present switching cycle of the secondary-side switch based on the reflected input voltage as measured at steady-state after the settling time lapses during the previous switching cycle comprises:
comparing the off time determined for the previous switching cycle of the secondary-side switch to a maximum off time calculated for the secondary-side switch; and maintaining the same error current for the present switching cycle of the secondary-side switch as the previous switching cycle if the off time determined for the previous switching cycle is the same as the maximum off time.

7 . The method of claim 1 , wherein adjusting the discharge current for the present switching cycle of the secondary-side switch based on the reflected input voltage as measured at steady-state after the settling time lapses during the previous switching cycle comprises:
sampling values of the reflected input voltage during the previous switching cycle of the secondary-side switch and holding one of the values sampled at steady-state; and using the value of the reflected input voltage sampled at steady-state during the previous switching cycle of the secondary-side switch in determining the off time of the secondary-side switch for the present switching cycle of the secondary-side switch.

8 - 10 . (canceled)

11 . A flyback converter, comprising:
a primary-side switch connected to a primary-side winding of a transformer; a secondary-side switch connected to a secondary-side winding of the transformer; a primary-side controller operable to control the primary-side switch to store energy in the transformer during ON periods of the primary-side switch; a secondary-side controller operable to:
switch on the secondary-side switch synchronously with switching off the primary-side switch to transfer energy from the transformer to the secondary side;
determine an off time of the secondary-side switch based on a reflected input voltage measured at the secondary-side winding when the primary-side switch is on;
account for a settling time of the reflected input voltage when determining the off time of the secondary-side switch, so that the settling time has little or no effect on the off time; and
switch off the secondary-side switch based on the off time;
a current source configured to provide a charge current for charging a capacitor on the secondary side when the primary-side switch is on; and a current sink configured to sink a discharge current for discharging the capacitor when the secondary-side switch is on, wherein the secondary-side controller is operable to adjust the discharge current for a present switching cycle of the secondary-side switch by an error current which is based on a comparison of the off time determined for a previous switching cycle of the secondary-side switch to a maximum off time calculated for the secondary-side switch.

12 . (canceled)

13 . The flyback converter of claim 11 , wherein the secondary-side controller is operable to switch off the secondary-side switch when the voltage across the capacitor decreases to a first reference value.

14 . The flyback converter of claim 11 , wherein the secondary-side controller is operable to:
compare the off time determined for the previous switching cycle of the secondary-side switch to a maximum off time calculated for the secondary-side switch; and decrease the error current for the present switching cycle of the secondary-side switch if the off time determined for the previous switching cycle is less than the maximum off time.

15 . The flyback converter of claim 1 , wherein the secondary-side controller is operable to:
compare the off time determined for the previous switching cycle of the secondary-side switch to a maximum off time calculated for the secondary-side switch; and increase the error current for the present switching cycle of the secondary-side switch if the off time determined for the previous switching cycle is greater than the maximum off time.

16 . The flyback converter of claim 11 , wherein the secondary-side controller is operable to:
compare the off time determined for the previous switching cycle of the secondary-side switch to a maximum off time calculated for the secondary-side switch; and maintain the same error current for the present switching cycle of the secondary-side switch as the previous switching cycle if the off time determined for the previo

Assignees

• Infineon Technologies Austria Ag

Inventors

• Gong Xiaowu
• Fahlenkamp Marc
• Sanders Anthony

Classifications

• H02M3/33592 Primary

  having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer · CPC title

• H02M3/33576 Primary

  having at least one active switching element at the secondary side of an isolation transformer · CPC title

• Y02B70/10

  Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes · CPC title